How can I add an attachment (pDF) to an invoice created in Dexxter?

Hello, I work as a freelancer through an agency. The agency uses accounting software that automatically generates my sales invoices based on the services I have provided. I receive these invoices by email in pDF format. The disadvantage of this is that my numbering is not stated. So I create my sales invoice myself in Dexxter, but I would like to add that pDF as an attachment. Is this possible? If yes, how?